Method and apparatus for strip-coating a metallic strip-shaped substrate with a plastic band and strip thus obtained

ABSTRACT

Method for strip-coating a metallic strip-shaped substrate ( 1 ) with a band ( 3 ) of plastic comprising the stages: (i) in-situ casting of a plastic band; (ii) leading the plastic band around a cooling roll ( 5, 5   a ); (iii) leading away the plastic band until the plastic band production is underway and stabilized; (iv) bringing the plastic band up to speed and heating the substrate to a temperature of the substrate close to the softening temperature of the part of the plastic band facing the substrate; (v) pressing the plastic band onto the substrate and where applicable breaking off the plastic band and stopping it being led away; (vi) coating the substrate ( 1 ) with the plastic band ( 3 ) at high speed.

The invention relates to a method for strip-coating a metallicstrip-shaped substrate with a thin strip of plastic, an apparatus forcarrying out the method as well as to the coated strip obtained with themethod.

There are at least two methods known for manufacturing a coated productcomprising a metal substrate and a plastic layer adhering to it, namelyfilm-laminating and extrusion-coating.

In the case of film-laminating, a finished plastic film is unrolled andapplied onto the metal substrate, as disclosed in e.g. WO 93/24324.

In the case of extrusion-coating a sheet of plastic is applied onto themetal substrate directly or virtually directly from an extruder, asdisclosed in e.g. EP 0 067 060 Al.

In the case of the first method a roll of finished film is taken as stagmaterial, A problem in making a roll of film is rolling it up. The filmtends to stick to itself so that the windings cling to each other.Because in its rolled up state the film shrinks somewhat the roll has tobe rolled up loosely to enable it to be unrolled once again in acontrolled way. Inevitable stresses in the film then easily cause edgebuild-up, the roll becomes unround, and the film displays spacing trackswhen being unrolled. Among other things this makes the film incapable ofbeing unrolled without difficulty at a sufficiently high speed; if isdoes succeed then there remains the problem that at higher rolling offspeeds electrostatic discharge symptoms need to be reckoned with. Toavoid such difficulties additives are added to for example householdfoils; in the case of film-lamination this solution offers no remedybecause the additives unacceptably reduce the capacity to adhere to themetal substrate.

On the face of it extrusion-coating would therefore seem an interestingalternative, and so it is for a small number of applications, namelythose whereby the plastic involved has the correct adhesion propertiesin molten state. When this is no longer the cases or not adequately so,and molecules need to be incorporated in the plastic to migrate to thesurface in order to accomplish adhesion, in the case ofextrusion-coating the problems occur, at least where a high coatingspeed is desired. This is because adhesion groups only migrate fastenough, i.e. within tenths of a second, if a sufficiently hightemperature can be maintained dig the coating. This is only possiblewhen coating onto one side of the substrate, The required hightemperature then also makes it impossible subsequently to coat the otherside because the previously applied coating becomes unacceptably damagedon the second exposure to the high temperature. Even non-subsequent butsimultaneous two-sided extrusion-coating is no solution because in thecase of extrusion-coating the slightest deviation in substrate thicknessand the slightest disturbance in the process would cause unstableprocess operation and consequently coating differences andinhomogeneities on each side.

The problems surrounding the procedures outlined are resolved or atleast largely reduced if worked in accordance with the invention.

The method in accordance with the invention is characterized in that itcomprises in combination the stages

(i) in-situ casting of a plastic strip;

(ii) leading the plastic strip around a preferably initiallywater-cooled cooling roll;

(iii) leading away the plastic strip until the plastic strip productionis underway and stabilised;

(iv) bringing the plastic strip and the substrate up to speed andheating the substrate to a temper of the substrate close to or above thesoftening temperature of the part of the plastic strip facing thesubstrate;

(v) pressing the plastic strip onto the substrate and where applicablebreaking off the plastic strip and stopping it being led away, while thesubstrate and the cooling roll are connected by the plastic strip;

(vi) coating the substrate with the plastic strip at high speed

This achieves the effect of enabling a considerably inner plastic layerto be applied onto the metal strip in a controlled and economicallyviable manner.

It is remarked that U.S. Pat. No. 5,407,702 discloses a method forcoating a metal strip with a polymer extrudate which extrudate afterextrusion is firstly brought into contact with a surface having atemperature which will promote sticking or clinging of the extrudatethereto. A typical temperature for this purpose is said to be in therange of about 120° C. to 180° C.

The invention is also embodied in an apparatus for the continuousstrip-coating of a metal substrate with a layer of plastic.

Finally the invention is further embodied in a strip-coated packagingsteel.

The invention will now be further illustrated by reference to thedrawing comprising FIGS. 1, 2 and 3 each of which show a possible linedrawing for coating in accordance with the invention, and severalnon-limitative examples with references to the Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified, diagrammatic line drawing of a preferredembodiment of the invention.

FIG. 2 is a diagrammatic line drawing of a preferred embodiment of oneside of the invention which shows additional features not shown in FIG.1.

FIG. 3 is a simplified, diagrammatic line drawing of a preferredembodiment of the invention which emphasizes the relative placement ofrollers and identifies the general direction of movement of a substrate.

EXAMPLE 1

An ECCS substrate (1) (also known as TFS) with a thickness of 0.20 mm.This substrate is heated to a temperature of 230° C. by means of heating(2), for example comprising heated guide rolls and/or on the basis ofinduction, hot air or otherwise. A plastic strip (3) such as a PETplastic strip is produced by applying on each side of substrate (1)molten PET via nozzle (4), (4 a) on an internally water-cooled guideroll (5), (5 a). The cooled PET strip (3) is then conveyed to the rubbercoated contact roll (6). As it travels it is possible to monitor thethickness, colour and strip tension and to trim to the correct width.The thickness of the two strips (3) is between 3 and 100 μm. Prior tocommencement of coating the contact rolls (6) do not touch substrate(1), and the two strips are conveyed off, for example rolled up onwinders (7).

In order to begin coating, contact rolls (6) are closed, i.e. movedtowards substrate (1).

PET strips (3) adhere to substrate (1) and most simultaneously thepieces of ship (3) between contact roll (6) and winder (7) are cutthrough. The rubber of rolls (6) is cooled externally, for example bymetal cooling roll (8), or by an air-blade on the rubber surface. Thecoated strip is then subjected to a brief extra heat treatment to 260°C. in order to optimise adhesion. At good product results, particularlysuitable for example for the covers of three-piece cans.

EXAMPLE 2

As in Example 1 but now on the one side of substrate (1) the plasticflowing from nozzle (4 a) is a two-layer polypropylene, whereby one ofthe layers, the adhesion layer, is maleic acid anhydride modifiedpolypropylene; on the other side of the substrate a PET strip ismanufactured and supplied via nozzle (4) and cooling roll (5). In thiscase the preheating temperature of substrate (1) is 200° C. Thethickness of the two strips is between 3 and 100 μm. During the initialcontact of the two-layer strip with the substrate at 200° C., atemperature above the melting temperature of polypropylene, there isalready some adhesion onto substrate (1), while the polypropylene toplayer neither sticks to or is damaged by the rubber of contact roll (6)that has a temperature of approx. 90° C. This adhesion to substrate (1)attains its maximum after approx. 1 second. The coated strip is thensubjected to a brief extra heat treatment, for example to 260° C., inorder to optimise the adhesion of both PET and modified PP. A productresults, for example particularly suitable for beer bottle crownclosures.

EXAMPLE 3

As in example 1 but now both plastic strips are two-layer polypropyleneSubstrate (1) is 0.10 mm thick ECCS and is heated to a temperature of230° C. Now a product results with a PP layer on both sides. The highertemperature than in example 2 is necessary because of the low heatcontent of thin substrate. The product is particularly suitable forexample for animal food packaging.

FIGS. 1, 2 and 3 show different line drawings in accordance with theinvention of coating a metal substrate with an in-line manufacturedplastic strip. FIG. 2 shows a vacuum chamber (20), an electrostatic edgelimiter (9), an air-blade (10) for cooling, a thickness gauge (11), anedging knife (12), a cutting waste extractor installation (13), atemperate gauge (14) and a furnace (15) for beating the coated strip.

It is possible to stretch the plastic strip at a temperature above theglass transition temperature and below the softening temperature of theplastic; in the case of uni-axial stretching an elongation of up to 400%is conceivable. If desired it is possible to provide the plastic stripwith openings.

To the expert it will be clear that the invention can be applied forsingle-side or two-side coating of a metallic substrate with on eachside the same plastic, or a different plastic for example PET orpolypropylene or on one side polypropylene and on the other side PET.

As seen in FIGS. 1-3 the substrate travels along a single straight linefrom the heating means 2, between the contact rolls 6, and to thefurnace 15 (FIG. 2). For example, FIG. 2 shows the heating means andcontact roll are arranged such that the metallic strip shaped substrateis straight immediately before, during and immediately after coating.

What is claimed is:
 1. A method for strip-coating a metallicstrip-shaped substrate with a strip of plastic comprising the successivestages of: (i) plastic strip production comprising in-situ casting of aplastic strip; (ii) leading the plastic strip around a cooling roll;(iii) leading away the plastic strip between an opened contact roll andthe substrate until the plastic strip production is underway andstabilized; (iv) bringing the plastic strip and the substrate up tospeed and heating the substrate to a temperature at or above thesoftening temperature of the part of the plastic strip facing thesubstrate; (v) pressing the plastic strip onto the substrate by closingthe contact roll and where applicable breaking off the plastic strip andstopping the plastic strip being led away, while the substrate and thecooling roll are connected by the plastic strip; and (vi) coating thesubstrate with the plastic strip; while performing on the plastic stripas the plastic strip travels between the cooling roll and the contactroll at least one of monitoring thickness of the plastic strip,monitoring color of the plastic strip, monitoring strip tension andtrimming width of the plastic strip.
 2. The method in accordance withclaim 1, wherein after the plastic strip has been applied an extra heattreatment stage follows to improve adhesion.
 3. The method of claim 1,wherein the cooling roll is internally water-cooled.
 4. The method ofclaim 1, further comprising incorporating adhesion-promoting moleculesinto the plastic strip, wherein the coating speed is high enough thatthe adhesion-promoting molecules must be capable of migrating to thesurface of the plastic strip within about one second after the plasticstrip contacts the substrate.
 5. The method of claim 1, furthercomprising stretching the plastic strip at a temperature above the glasstransition temperature of the plastic strip.
 6. The method of claim 1,further comprising uniaxially stretching the plastic strip at most 400%at a temperature above the glass transition temperature of the plasticstrip.
 7. The method of claim 1, wherein the metallic strip shapedsubstrate travels along a single straight line from the heating means toa furnace downstream of the contact roll.
 8. The method of claim 1,wherein the metallic strip shaped substrate is straight immediatelybefore, during and immediately after coating.
 9. The method of claim 1,wherein the metallic strip shaped substrate is straight during coating.10. The method of claim 1, wherein in step (iv) the substrate is heatedto a temperature above the softening temperature of the part of theplastic strip facing the substrate.
 11. The method of claim 1, whereinin step (iv) the substrate is heated to a temperature in the range from200 to 230° C.
 12. The method of claim 1, wherein the metallic strip hasa first side and a second side opposed to the first side and the firstside of the metallic strip shaped substrate is coated with said strip ofplastic and simultaneously the second side of the metallic strip shapedsubstrate is coated with a second strip of plastic, wherein saidstrip-coating of said second side of said metallic strip-shapedsubstrate with said second strip of plastic comprising the successivestages of: (i) plastic strip production comprising in-situ casting ofsaid second plastic strip; (ii) leading the second plastic strip arounda second cooling roll; (iii) leading away the second plastic stripbetween an opened second contact roll and the substrate until the secondplastic strip production is underway and stabilized; (iv) bringing thesecond plastic strip and the substrate up to speed and heating thesubstrate to a temperature at or above the softening temperature of thepart of the second plastic strip facing the substrate; (v) pressing thesecond plastic strip onto the substrate by closing the second contactroll and where applicable breaking off the second plastic strip andstopping the second plastic strip being led away, while the substrateand the second cooling roll are connected by the second plastic strip;and (vi) coating the substrate with the second plastic strip; whileperforming on the second plastic strip as the second plastic striptravels between second cooling roll and second contact roll at least oneof monitoring thickness of the second plastic strip, monitoring color ofthe second plastic strip, monitoring strip tension and trimming width ofthe second plastic strip.
 13. The method of claim 12, wherein in step(iv) the substrate is heated to a temperature in the range from 200 to230° C.
 14. An apparatus for strip-coating a metallic strip-shapedsubstrate with a strip of plastic in accordance with claim 1, comprisingin combination: means of conveying the metallic strip-shaped substrate;a contact roll for pressing the plastic strip onto the substrate; meansof producing the plastic strip comprising means of casting for castingthe plastic; a cooling roll for the formation of a plastic strip; meansof feeding and guiding for bringing the plastic strip to the substratevia the contact roll and for leading away the plastic strip between anopen said contact roll and the substrate until the plastic stripproduction is underway and stabilized; wherein the contact roll ismoveable to a first position apart from the substrate wherein thecontact roll is suitably arranged to co-operate with a means ofconveying off the plastic strip and to a second position relative to thesubstrate wherein the contact roll is suitable to press the plasticstrip onto the substrate.
 15. The apparatus in accordance with claim 14,wherein the means of conveying substrate, the contact roll, the means ofcasting, the cooling roll and the means of feeding and guiding areessentially duplicated, one set on each side of where the substrate issituated during operation for simultaneously two-sided coating themetallic strip shaped substrate.
 16. The apparatus of claim 14, furthercomprising a furnace downstream of the contact roll, wherein the heatingmeans, contact roll and furnace are arranged such that the metallicstrip shaped substrate travels along a single straight line from theheating means to the furnace downstream of the contact roll.
 17. Theapparatus of claim 14, wherein the heating means and contact roll arearranged such that the metallic strip shaped substrate is straightimmediately before, during and immediately after coating.
 18. Theapparatus of claim 14, wherein the heating means and contact roll arearranged such that the metallic strip shaped substrate is straightduring coating.
 19. The apparatus in accordance with claim 14, whereinthe contact roll is rubber at least on a surface with which it comesinto contact with the plastic band.
 20. The apparatus in accordance withclaim 19, wherein the means of conveying substrate, the contact roll,the means of casting, the cooling roll and the means of feeding andguiding are essentially duplicated, one set on each side of where thesubstrate is situated during operation for simultaneously two-sidedcoating the substrate.